RNA polymerase III (Pol III) and transcription factor IIIB (TFIIIB) assemble together on different promoter types to initiate the transcription of small, structured RNAs. Here we present structures of Pol III preinitiation complexes, comprising the 17-subunit Pol III and the heterotrimeric transcription factor TFIIIB, bound to a natural promoter in different functional states. Electron cryo-microscopy reconstructions, varying from 3.7 Å to 5.5 Å resolution, include two early intermediates in which the DNA duplex is closed, an open DNA complex, and an initially transcribing complex with RNA in the active site. Our structures reveal an extremely tight, multivalent interaction between TFIIIB and promoter DNA, and explain how TFIIIB recruits Pol III. Together, TFIIIB and Pol III subunit C37 activate the intrinsic transcription factor-like activity of the Pol III-specific heterotrimer to initiate the melting of double-stranded DNA, in a mechanism similar to that of the Pol II system.

中文翻译：

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RNA聚合酶III打开启动子的分子机制

RNA 聚合酶 III (Pol III) 和转录因子 IIIB (TFIIIB) 在不同的启动子类型上组装在一起，以启动小型结构化 RNA 的转录。在这里，我们展示了 Pol III 预启动复合物的结构，包括 17 个亚基 Pol III 和异源三聚体转录因子 TFIIIB，与处于不同功能状态的天然启动子结合。电子冷冻显微镜重建，分辨率从 3.7 Å 到 5.5 Å 不等，包括两个早期中间体，其中 DNA 双链体是封闭的，一个开放的 DNA 复合体，以及一个在活性位点具有 RNA 的初始转录复合体。我们的结构揭示了 TFIIIB 和启动子 DNA 之间极其紧密的多价相互作用，并解释了 TFIIIB 如何招募 Pol III。一起，